1. Field of the Invention
The invention is in the field of radar ranging and more particularly in the field of error detection and correction in plural frequency ranging radar systems. The invention has application, for example, in guidance systems, laser surveying and automatic camera focusing apparatus.
2. Description of the Prior Art
Radar ranging devices are based upon measurements of phase differences between transmitted and received signals. Such systems suffer from measurement ambiguities resulting from the fact that phase is a multivalent function. In using high pulse repetition frequencies (PRF) in which the target range is greater than the equivalent distance between transmitted pulses, it is uncertain as to which transmitted pulse with which the received pulse is to be correlated. Multiple pulse repetition frequencies have been utilized to modulate the transmitted signal in order to associate the return signal with the proper transmitted signal thus removing the ambiquity. Multiple, fixed PRF's have been used to obtain accurate range data wherein sequential measurements of the ambigious range corresponding to each PRF are obtained and compared to obtain the common range bin defining the range unambigiously. The PRF's are chosen to have a common submultiple frequency. Range uniqueness is assured for certain pairwise relatively prime harmonics of the counting frequency by the Chinese Remainder Theorem.
Reference is made to several prior art references, incorporated herein by reference, namely Radar Handbook, by M. Skolnik, McGraw-Hill (1970), Chapter 19 and especially Section 19.3 and U.S. Pat. Nos. 3,277,473, 4,106,019 and 3,649,123.
In making the range measurement for each PRF, small errors in any of the measured phase shifts can give rise to catastrophic errors in the synthesized or decoded range. Wide objects which fill several range gates can also yield several valid phase measurements for each frequency, and thus yield widely different (and incorrect) decoded ranges. Such range measurement errors develop not only in PRF systems, but in systems using other forms of modulation such as amplitude modulation, pulse-width modulation, and frequency modulation. The present invention uses amplitude modulation as an exemplary embodiment.